The title compound, C45H55NO6P2, consists of an acyclic P=N-P(O) monophosphazene chain and five bulky 2,4,6trimethylphenoxy side groups which predominantly determine the molecular shape. Although the P-N single [1.586 (3) Å] and P=N double [1.517 (3) Å] bonds are significantly different from each other, both are substantially shorter than the ideal P-N single bond. The P-N-P angle [146.0 (2)°] corresponds to the upper limit reported for acyclic phosphazene derivatives in the literature. © 2000 International Union of Crystallography.